Interference rejection combining (IRC) [2-8] is a method to enhance the transmission capacity in a communication system by mitigating undesirable co-channel/adjacent interference. This is made possible by estimating and utilizing the spatial correlation of the interfering signals between multiple receiving antenna elements of the victim receiver. In doing so, the received interference is suppressed by spatial whitening. In the application of communication, the IRC is typically followed by receiver equalization and decoding. An alternative to IRC is maximum ratio combining (MRC) of the antenna signals. The MRC criterion serves to maximize the signal-to-noise ratio (SNR) rather than maximizing signal-to-interference-plus-noise-ratio (SINR).
A multi carrier system e.g. LTE (Long Term Evolution) employs reference symbols transmitted at known time/frequency resources, i.e., known pilot symbols, from which the SC-FDMA/OFDM (Single Carrier Frequency Division Multiple Access/Orthogonal Frequency Division Multiplexing) receiver can estimate the channel and the spatial covariance matrix of the interference plus noise.
For LTE UL (UpLink) user signals are allocated in the frequency domain by one or more groups of 12 contiguous sub carriers, i.e. one or more resource blocks (RB:s). For LTE DL (DownLink) the RB:s are also allocated in the frequency domain but not necessarily contiguously. Hence, for both UL and DL the LTE cell-interference tends to be frequency-dependent. Moreover, adjacent interference due to e.g. leakage from neighboring systems typically interfere more at the frequency band edges. Thus, also the adjacent interference tends to be frequency-dependent.
IRC can typically be viewed as spatially whitening the received signal before further processing and combining the antenna signals according to e.g. the MRC algorithm. The coefficients of the whitening filter are typically calculated from the estimated wideband residual noise, e.g. the spatial covariance matrix of the interference plus noise. Different criterions such as the minimum means-square error (MMSE) and the optimum combining (OC) have been proposed to compute the IRC coefficients [1].
A frequency-domain based IRC approach, described in e.g. [1] (referred to as “digital beam forming in the frequency domain”), transforms the broadband received signal into the frequency domain with a Fast Fourier Transform (FFT) and performs IRC (or beam forming) per frequency bin, followed by inverse Fast Fourier Transform of the combined signal. The IRC coefficients are in this case selected by independently minimizing the mean output power at each frequency bin.
The major drawback with previously described IRC methods [2-8] for multi-carrier based systems, as e.g. LTE, are the high computational complexity associated with calculating the IRC coefficients, and the reduced performance in case of wideband IRC with frequency-dependent interference.
More specifically, the disadvantages with employing wideband IRC for LTE are the following:                With strong frequency-dependent interference, e.g. cell-interference/co-channel interference, the optimal whitening filter is a function of frequency, and hence, the receiver performance with wideband IRC is typically reduced compared to frequency-dependent IRC.        
The disadvantages with employing frequency-bin based IRC are the following:                The computational complexity associated with estimating the frequency-dependent whitening filter and the frequency-dependent spatial noise covariance matrix are significant and grows strongly with the number of sub carriers and antennas.        
There is therefore a need for a solution to the above mentioned disadvantages.